The invention relates to a weldable strain sensor and more particularly to a weldable strain sensor suitable for curved surfaces.
Strains of material surfaces can, e.g., be measured with strain sensors, which are mounted on these surfaces. As the surface is stretched or compressed, the strain sensors are also impacted, so that a measurement signal is generated.
Weldable strain sensors are known from the prior art. For this purpose, the documents DE 2658324 C2, JP 2003090772 A, JP 4519703 B2 and JP 5378778 B2 are mentioned as examples.
In most cases, so-called strain gauges are used as strain sensors, which are fastened by means of an adhesive. Adhesives can easily be processed, but have also properties that can be detrimental under certain environmental conditions. These unfavorable environmental conditions involve greatly fluctuating or high humidity and in particular large temperature differences. In addition, there is the following problem: Adhesives can be optimally processed only at so-called room temperature. When, e.g., strain sensors should be secured to steel girders of a railway bridge or to gas pipelines, this becomes then basically impossible, when the ambient temperature is below 0 degree Celsius for example. In principle, it is also not possible to heat the measuring point, because this causes temperature-induced material expansions which would greatly falsify the measured values in this area.
Strain gauge strips include essentially a carrier material and the strain sensor itself. The strain sensor is firmly connected to the carrier material. Carrier materials that are attached by means of adhesives are usually thin and therefore very flexible plastic films. Weldable carrier materials are usually steel sheets which are fastened by spot-welded joints on the surface to be examined.
An important difference between bondable strain gauges and weldable strain sensors is the number of work steps in the so-called application. The application of strain sensors must be carried out by specialists, because even a single improperly executed work step can lead to a short or medium-term failure of the measuring point. Hard to reach measuring points, such as railway bridges or gas pipelines, thus require considerable effort when repair of a measuring point is involved. It is known that an increase in the number of application steps also increases the error probability. When an application requires 10 work steps for example, and only one of these work steps is not executed properly, a faulty application is caused, i.e. a measuring point with impermissibly high measuring error or high failure probability. When, however, fewer steps are required for an application, the error probability thus also decreases.
In the past, application sites were usually provided with so-called protective covers for protection against moisture and mechanical damage. However, attaching these protective covers in turn increases the number of application steps and thus also the likelihood of improperly executed application steps.
It is thus desirable to provide a fastening technology with least possible error probability. Since the strain sensors are already encapsulated during manufacture, welding per se requires little effort. However, these capsules are mechanically stiff and cannot be attached to curved surfaces. A solution to the problem would be the production of encapsulated strain sensors with capsules that already have the radius of the workpiece surface to which the strain sensor is to be welded. However, for each radius of a workpiece surface, a special strain sensor must then be made. Therefore, this technique has not prevailed in practice.